1. Field of the Invention
This invention relates to electrofusion plastic fittings, and more particularly, to improved heat-shrinkable electrofusion fittings for sealingly connecting the ends of pipes or tubular fittings.
2. Description of the Prior Art
A variety of electrofusion plastic fittings have heretofore been developed and used for joining the ends of plastic pipes or other tubular plastic fittings such as tees, ells, crosses and the tubular connecting ends of plastic valves and the like which are adapted to be connected in plastic piping systems. Some of such electrofusion fittings include electric resistance heating coils disposed therein. When an electric current is applied to the coil of such a fitting, the plastic material making up the fitting adjacent to the coil is melted and the fitting is sealingly fused with one or more complimentary plastic fittings or pipes. Other electrofusion fittings have been developed and used which include particulate ferromagnetic and/or other conductive fillers. The fillers cause the entire fittings to be heated when the fillers are electrically energized.
In order to allow the easy assembly of an electrofusion fitting with a complementary plastic fitting, space must exist between the surfaces of the fittings to be fused. In the smaller sizes of fittings, the space is filled by the volumetric expansion of plastic material as it changes from the solid phase to the liquid phase. That is, in smaller fittings, a sufficiently large volume of plastic material undergoes a phase change to fill the space between the fittings during the heating cycle and still allow sufficient time for good bonding to take place before the end of the fusing process. In large diameter fittings, the space is larger and obtaining a good fused joint is more difficult. A further problem with large diameter fittings is the separation of the layers on either side of the interface between the fittings due to shrinkage of the molten region as it cools.
Electrofusion fittings have heretofore been heat-shrinkable so that when the fittings are heated, they advantageously shrink to a smaller size and exert clamping pressure on the complimentary fitting or fittings to be fused. While such heat-shrinkable fittings solve some of the above mentioned problems, in order to fuse such fittings they must be heated to above their melting temperatures which makes the fusing process difficult to control. In order to overcome those problems, inserts formed of lower temperature fusable plastic materials have been placed between the shrinkable fittings and the complimentary fittings to which they are to be fused. However, the inserts must be heated by heat from the shrinkable fittings which again makes the fusing process difficult to control.
Another problem which is common to all electrofusion fittings involves the oxidization of the surfaces of the complimentary fittings to be fused. After a plastic pipe or fitting is exposed to the atmosphere for a period of time, its outer surfaces become highly oxidized which inhibits the fusing process. That is, unless the outer surface of the complimentary fitting is thoroughly scraped before fusion, a good fused joint will very often not be obtained.
Thus, the prior art electrofusion fittings all suffer from one or more disadvantages, especially in obtaining good fused joints between the electrofusion fittings and complimentary pipes or fittings. Accordingly, there are continuing needs for improved electrofusion fittings and methods.